Yazid bin Bilal narrates from Sayyiduna Ali (may Allah be pleased with him): The fasting person should not use the miswak in the evening, but at night, for the dryness of the fasting person's lips will be a light between his eyes on the Day of Resurrection.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8334